The present invention relates to a failure detection for an internal-combustion engine in which exhaust gas recirculation is applied.
Conventionally, an intake air amount into an engine is controlled by returning a part of the exhaust gas to the intake air (this approach is referred to as “exhaust gas recirculation (EGR)”). Since the oxygen density in the intake air decreases when the EGR is applied, a combustion temperature becomes lower, contributing to reduction of the generated NOx emission.
Japanese Patent No. 3097491 discloses a technique for detecting a clogging in a recirculation passage in an engine having an EGR mechanism based on a relation between an engine operating condition and a significance of a pulsation of an output from a pressure sensor disposed in the recirculation passage.
However, this conventional technique cannot detect such leakage failure wherein the outside air flows into the recirculation passage due to occurrence of a crack or a disconnection in the recirculation passage. Under the condition where the leakage is occurring, the exhaust gas containing more external air recirculates into the engine than the re-circulating exhaust gas at no leakage failure time. Therefore, the oxygen density of the intake air becomes higher than normally. When a fuel amount corresponding to the normal recirculation amount is injected under such a condition, the air-fuel ratio becomes lean temporarily, that is, the air-fuel ratio goes out of the optimum range of the catalyst purification rate. As a result, the emission after passing through the catalyst becomes bad.
Accordingly, it is an objective of the present invention to resolve the above-described problem.